726 Introduction to Machine Guarding

Photoelectric Presence-Sensing Devices

A photoelectric (optical) presence-sensing device uses a system of light sources and controls that can interrupt the machine's operating cycle.

Key Features of Photoelectric Presence-Sensing Devices

The key features of a photoelectric presence-sensing device include the following:

• If the light field is broken, the machine stops and will not cycle.
• It must be used only on machines that can be stopped before the worker can reach the danger area.
• The design and placement depend on the time it takes to stop the machine and how fast the operator’s hand can reach from the guard to the danger zone.

Examples

• Light curtains are vertical arrays of infrared light beams that stop the machine when any beam is interrupted.
• Laser scanners that scan an area with laser beams and stop the machine if the beam is broken.
• Optical sensors on press brakes that detect if a hand enters the danger zone and stop the press immediately.
• Safety light barriers on robotic arms that prevent operation if a person crosses into the robot's work area.
• Photoelectric sensors on conveyor systems that stop the conveyor if a worker's hand or body interrupts the light field near dangerous pinch points.

The knowledge and skills that employees must gain when using presence sensing devices are summarized in 1910.217 Subpart O and OSHA Publication 2254, Training Requirements in OSHA Standards .

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